THE HEAD PROBLEM 



619 



In the light of the evidence we are compelled to add another segment 

 to those enumerated by van Wijhe, so as to make eleven segments in the 

 elasmobranch head, instead of the ten recognized by Goodrich. 



F ^ Yi w 



in 



I ! ■- AC USflCO-FACIALlS GANGLION . 

 3Ssr7SB»cJs8«4Sfe*Ci:^ ' f-SOM ITE . 



Fic;. 512. — A parasagittal section of a 33-hour chick embryo showing the brain neuro- 

 meres. The neuromeres are indicated by Roman numerals. 



Among the criteria used to determine head segmentation, some 

 morphologists have leaned heavily upon the so-called neuromeres or 

 segments of the central nervous system. Neuromeres are evanescent 



'NEURAL CREST CELLS/ 

 NEUROMERES 



Fig. 513. — A reconstruction of the head region of a 5 mm. Squalus embryo showing 

 neuromeres and van Wijhe's somites as viewed from the left side. Neural crest cells 

 are indicated by heavy black dots. Roman numerals number the neuromeres. 



embryonic structures, first described by Bischoff ('42) in the medulla 

 of mammal embryos. The assumption that neuromeres are reliable 

 criteria of segmentation has been based partly upon the belief that 

 vertebrates are the descendents of annelids which have a segmented 



